CSO Advisory: Atmospheric river leads to combined sewer overflow to the Willamette River

This advisory expired at 3 pm on December 11, 2025.

About the CSO

• The CSO affects the stretch of the Willamette River downstream of the Ross Island Bridge.
• The overflow began at 3:15 am on December 9 and ended around 3 pm on December 9.
• The entire event lasted about 11 hours and 45 minutes.
• Data is still being gathered on the total volume discharged to the Willamette River.

Public advisory

• Avoid contact with the river in the affected area during the overflow and for 48 hours after it stops due to increased bacteria levels.
• Recreational activities such as swimming and boating are not advised during this time.

About CSOs

• A CSO is approximately 80% stormwater and 20% sewage.
• Overflows can occur during periods of heavy rain or snowfall.
• Portland's Big Pipe Project has reduced CSOs by 94% to the Willamette River and 99% to the Columbia Slough.
• Overflows used to occur 50 times a year, sometimes lasting for days, but now occur 4 times a year on average.

Stay updated and learn more

• Track the overflow or how the Big Pipes are keeping combined sewage out of the Willamette River in real time by following the Big Pipe Tracker.
• Learn more about CSOs and the Big Pipe Project.